Lecture 31: Hemodynamics Flashcards
Two major determining factors of flow
- Pressure gradient along tube
- Resistance
As change in pressure increases
- Flow increases
As resistance increases
- Flow decreases
Flow in vasculature
- Units are mL (or L)/min and Q is symbol
- Flow in systemic or pulmonary circuit is amount of blood pump per minute by right or left ventricle
- For either circuit Q is the cardiac output, the amount of blood pumped by heart per minute
- Normal Q at rest is about 5 L/min
Blood pressure units
- Usually mmHg
- Sometimes cmH20 used (1 mmHg = 1.36 cmH20)
Resistance
- Cannot be measured, but calculated by:
- Q = DeltaP/R
- R = DeltaP/Q
Poiseuille’s Law assumptions
- Newtonian fluid
- Laminar flow
- Non-pulsatile
Laminar flow
- Turbulent flow has greater resistance than laminar flow
- Reynolds number can be used to predict turbulence results in noise (hence murmurs) or, if in heart, heart sounds
Advantage of parallel arrangement of arteries, arterioles, etc.
- Independent flow regulation
- Identical blood composition
- Reduces workload of heart
Due to distensibility of blood vessels, increasing
pressure
- Increases blood vessel diameter
- Thereby, decreasing resistance and vice versa
Effects of changes in arterial pressure on blood flow (normal curve)
- Flow decreases rapidly at first from 130 mmHg down due to:
- Decrease in A-V pressure difference
- Decrease in vessel diameter
As arterial pressure decreases, the rate of flow
- Decreases exponentially
- Slope also decreases
At 20 mmHg flow ceases entirely, called the
- Critical closing pressure
- At this point, the arterioles close completely
- No flow to tissues
Law of Laplace
- Circumferential force (F) tending to stretch muscle fibers in vascular wall (F) is proportional to vessel radius (r) times the pressure (P)
- Inversely proportional to the wall thickness (t)
Law of Laplace equation
- T is proportional to (Pr/t)
During vascular strecthing as P decreases,
- “r” also decreases (since it is dependent on P)
- Force (F) maintaining stretch decreases rapidly
- When elastic tension in wall becomes greater than the stretching caused by pressure, the vessel closes
Critical closing pressure in the presence of RBCs
- Keep critical closing pressure higher
- 20 mmHg for whole blood
- (-)5 to (-)10 mmHg when plasma only flowing
Factors that regulate resistance
- Viscosity
- Length
- Radius
Viscosity effects
- Increasing hematocrit from 45 to 70% (as in severe polycythemia) doubles viscosity, which doubles resistance
- More than resistance increase seen in all hypertension (except most severe forms)
Vascular distensibility and compliance (capacity)
- Diameter increases as internal pressure increases (because blood vessels are elastic)
- Degree of elasticity varies from one type of vessel to another
Units of distensibility
- Fractional increase in volume for each mmHg rise in pressure
Veins are 6 -10 times as distensible as arteries; therefore, a given rise in pressure will cause
- 6 to 10 times as much blood to fill vein as an artery of comparable size
Veins have high capacitance, and arteries are not distensible so
- Veins store blood volume
- Arteries have low volume storage ability
Pressure volume curves
- Steep for arteries
- Small increases in volume cause large increases in pressure and vice versa
- Distensible veins: much larger increases in volume cause minimal changes in pressure
